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Types of Thermal Insulation - Which Contains Asbestos?
By Bill Kibbel


Vermiculite is the most common residential type of thermal insulation that might contain asbestos. It is a mica-like mineral that expands like popcorn when heated. Much of it installed between 1923 and 1990 could be contaminated with asbestos.

About 80% of all vermiculite produced before 1990 was from a mine near Libby, Montana. There is also a natural deposit of asbestos in this mine resulting in vermiculite that is contaminated with asbestos fibers. The Zonolite Company produced the vermiculite until it was purchased by WR Grace & Co in 1963.

If you find you have this type of insulation, it's probably not useful having it tested for asbestos. According to the EPA and CDC, there are significant limitations in testing methods for this particular product and test results may not accurately determine if any vermiculite sample contains asbestos. The sample collection methods used by those selling tests on this product is also questionable.

Since much of the vermiculite installed before 1990 came from the contaminated Montana mine, it's best to treat it the same as known asbestos containing materials. Most importantly, avoid anything that could disturb the insulation or distribute the fibers throughout a building.

Mineral Wool, Rock Wool, Slag Wool Balsam Wool & Silva Wool are NOT known to contain asbestos.

Rock wool is made from mineral fibers manufactured from waste from mining and volcanic rock. The materials are melted in furnaces and blown with air or steam over spinning drums or a centrifuge to create the fibers (picture making cotton candy).

Slag wool is made by the same precess from fibers created from blast furnace slag.
The above are generically referred to as mineral wool and since they are man-made fibers, they do not contain asbestos (asbestos is a naturally occurring fiber). I've read about mineral wool, asbestos and resins mixed together to manufacture a couple specific industrial insulating products for process equipment, but it's not the kind of stuff used for residential insulation.

Balsam wool and Silva wool are shredded wood products, some treated with borax as a fire-retardant. They're considered natural products and also are not known to contain asbestos.

Kimsul, looks like layered creped paper and is impregnated with asphalt. It is not known to contain asbestos. It was manufactured from the early-mid 1930s through the early 1950s. It was first used in refrigerators, then car dashboards and train cars. It was also used by the US military during WWII and lined many Quonset huts assembled in cold climates.

Asbestos Pipe Insulation

Asbestos pipe insulation is most commonly found on steam or gravity hot water heating pipes. It is occasionally used on water supply pipes and on some drain pipes in unconditioned spaces.
Asbestos Duct Insulation

Most asbestos duct insulation is a paper-like wrap around heating ducts. Later duct systems may have it wrapped only around the joints of the ducts.


Bill Kibbel, an expert in historic building materials and methods, is a historic building inspector & consultant.
2004, Wm. Kibbel III

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